Pipettes are used for liquid dosage in laboratories, the pipettes comprising a piston movable in a cylinder by means of an electric motor, by which piston liquid is aspirated in accordance with selected pipette function into and out of a tip connected to the cylinder. The liquid volume is usually adjustable. Pipettes comprise a control system and its user interface for i.a. setting of the volume and other necessary adjustments and for giving commands for performing operations. The user interface has the necessary push buttons for this purpose. The user interface also has a display by means of which i.a. the volume and other necessary data can be displayed. The display can also show menus by means of which the functions can be selected and settings fed using the push buttons.
The pipetting functions to be used are e.g. direct, reverse and step pipetting. Direct pipetting involves aspiration of a desired volume into the pipette and discharge of the volume. Reverse pipetting function involves aspiration of a volume greater than the one desired into the pipette, with the desired volume being subsequently discharged. Step pipetting involves aspiration of a volume into the pipette, the volume being subsequently discharged in a plurality of minor portions.
The piston has a basic position from which it can be moved upwards to an upper position. The distance between these positions defines the dosable volume. In addition, the piston has a lower position to which the piston can be moved downwards from the basic position by the length a so-called secondary movement. In direct pipetting the piston is moved from the basic position to the upper position when aspirating liquid and from the upper position to the lower position when discharging liquid. The secondary movement thus ensures that the liquid is discharged as completely as possible. In reverse pipetting the piston is moved from the lower position to the upper position when aspiring the liquid and from the upper position to the basic position when discharging the liquid. In step pipetting the piston is moved from the lower position to the upper position when aspirating the liquid and from the upper position step to the basic position when discharging the liquid. In step pipetting the excess volume aspirated by the secondary movement specifically ensures that also the last dosage to be discharged is full. In reverse pipetting and step pipetting the volume corresponding to the secondary movement is usually thrown away after pipetting. In known electronic pipettes the length of the secondary movement is constant.
Publication U.S. Pat. No. 3,343,539 discloses a dispensing device corresponding to a manually operated pipette in which also the length of the secondary movement is adjustable. Publication FI 44 070 discloses a manually operated pipette in which also the length of the secondary movement is adjustable, specifically the same distance as the primary movement and together and simultaneously with the primary movement.